[gmx-users] form of the angle potential in ffgmx2bon.itp

[Anton Feenstra]

Markus O Kaukonen wrote:
> 
> A) 0.5*397.480*(current_angle_in_deg - 109.500)^2  [kJ/mol]
[...]
> I get the impression from the manual 3.1.1 that it should be option A) but
> test calculations give strange results.

It's actually none of the above. Your option A is the *Gromos* angle
potential, but that is another function type. Function type 1 (4th
column in the ff*bon.itp [angletype] section) is the *Gromacs* angle,
which is (also defined in the manual):

E(alpha) = 0.5 * C_theta * cos(alpha - theta_0)^2

i.e. the quare of the cosine of the angle difference, and not the
square of the difference itself.


-- 
Groetjes,

Anton
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|        | Anton Feenstra                                            |
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